Video processing apparatus and method for detecting a temporal synchronization mismatch

ABSTRACT

A video processing apparatus and a method for detecting a temporal synchronization mismatch between at least a first and a second video stream of 3D video content are provided. A motion vector is determined for a group of pixels in consecutive frames of the left or the right video stream. Further, a disparity between the left and the right video stream is determined for the group of pixels, wherein the motion vector and the disparity are determined for a first number of frames and a subsequent second number of frames of the left and the right video stream.

FIELD OF THE INVENTION

The invention relates to a method for detecting a temporalsynchronization mismatch between at least a first and a second videostream of 3D video content. Further, the invention relates to a videoprocessing apparatus for detecting a temporal synchronization mismatchin 3D video content.

BACKGROUND OF THE INVENTION

In 3D-video, each eye of the viewer receives its own stream of images.Each image pair in the stream represents the same scene from a slightlydifferent perspective, creating a 3D experience in the human brainduring reproduction. Typically, a pair of synchronized cameras is usedfor capturing stereoscopic 3D video content. One camera captures theimages for the left eye, while the other camera captures the images forthe right eye. In this context, 3D-video content includes stereoscopicand multi-view video content and is also referred to as stereoscopicvideo content.

An object in the real word is projected onto different positions withinthe corresponding camera images. If the parameters for the stereo camerasetup are known and the displacement between corresponding points in thestereo images belonging to one and the same object in the real word canbe determined, the distance between the real world object and the stereocamera equipment, i.e. the depth of the object, may be calculated bytriangulation. The displacement between corresponding points in thestereo images is commonly referred to as disparity.

To produce high quality 3D video content, the stereo cameras must betightly synchronized so that each pair of images, i.e. the image orframe taken by the left camera and a corresponding image or frame takenby the right camera, are taken at the same moment in time. Otherwise,camera motion and moving objects in the captured scene will lead toadditional erroneous disparities.

Human observers are well known to be very sensitive to even smallamounts of any vertical disparities, which are by definition erroneous.However, altered or erroneous horizontal disparities can also lead tosevere distortions in reproduction of 3D video content. Further, anerroneous disparity between a left and a right picture can lead toconflicts between monocular occlusions and stereoscopic placement cuesas well as hyper-convergence or -divergence. These issues can easilylead to an unpleasant viewing experience similar to erroneous verticaldisparities, especially as motion in films tends to be more pronouncedin the horizontal direction.

In order to provide tight camera synchronization, stereo cameras areusually equipped with a “genlock” or “sync” input through which acentral timing generator unit can send a common sync-signal to each ofthe cameras to trigger the two capturing processes in a synchronousmanner. Nevertheless, a lot of 3D-content suffers from insufficientsynchronization. The reasons are manifold and range from hardwarefailures and tolerances to operator mistakes and editing errors.

As a consequence, proper synchronization in the final stereoscopic videocontent is one critical area to take care of when producing high quality3D-video content. According to the prior art, quality inspection withrespect to synchronization errors is performed manually, in most cases.However, this is a costly and time consuming process because the 3Dvideo content has to be inspected by an operator and the synchronizationmismatch has to be determined manually.

Document U.S. Pat. No. 6,340,991 B1 relates to a camera system and amethod for synchronization of video frames of a moving object capturedfrom a plurality of video cameras. A first mathematical model of themotion of said object is derived by processing a sequence of videoframes of the left channel of stereoscopic video content. A secondmathematical model of the motion of said object represented by a secondsequence of video frames of the right channel is derived by processingthis second sequence of video frames. The first mathematical model iscompared to the second mathematical model in order to calculate a timedifference between the left and right channel. However, estimation ofsynchronization mismatch based on mathematical models of motion isrestricted to rigid objects showing no deformations. Further, thismethod suffers from low accuracy and demands for high computationaleffort.

Accordingly, there is a need for a more efficient, automatic orsemi-automatic inspection, allowing detecting a synchronization mismatchin 3D-video content.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved video processingapparatus and an improved method for detecting a temporalsynchronization mismatch between at least a first and a second videostream of 3D video content.

In one aspect of the invention, a method for detecting a temporalsynchronization mismatch between at least a left and a right videostream of 3D video content is provided. A motion vector for a group ofpixels is determined in consecutive frames of the left and/or rightvideo stream. A group of pixels may be an arbitrary number of pixelsranging from a single pixel to all or nearly all pixels of therespective frame. For example, the determination of a motion vector maybe performed based on a point to point correspondence between frames ofthe left and/or right video stream. Further, a disparity between theleft video stream and the right video stream is determined for saidgroup of pixels. The motion vector and the disparity are determined in afirst number of frames and in a subsequent second number of frames.Within the context of the specification, a number of frames may be anarbitrary number of frames in principle. However, at least twoconsecutive frames per number of frames will be necessary fordetermination of a motion vector. For a more accurate determination ofmotion vectors, it is preferable to analyze a plurality of frames. Themotion vector in the first number of frames is compared with the motionvector in the second number of frames. Upon detection of a variation ofthe motion vector, the disparity which has been determined for the firstnumber of frames is compared with the disparity which has beendetermined for the second number of frames. A deviation of the disparityvalue in the first number of frames and the disparity value in thesecond number of frames is indicative to the synchronization mismatch.

If cameras of a stereo rig are not perfectly synchronized, the cameraswill capture a moving object at different moments in time and atdifferent positions along its trajectory in the real world. Somethingsimilar applies to a moving camera rig. For this reason, thedisplacement of objects or even points in the left and right videostream of 3D video content which is due to the physical offset of thecameras, in other words the true disparity between the left and rightpicture of a pair of stereo frames, will be superposed by a displacementin the direction of movement. These erroneous disparities areproportional to the apparent speed of an object or feature point as itis visible in the camera images and the synchronization offset.According to aspects of the invention, the temporal displacement withinone camera view is compared to the displacement (i.e. the sum of thetrue and the erroneous disparity) between the different camera views,i.e. between the different frames which are captured by the left andright camera of the stereo rig.

However, in order to eliminate the influence of misalignments of thestereo camera rig as well as of the true disparities, a change in motionwill be compared with the corresponding change in disparity. Thesynchronization offset may be determined for a moving object or for amoving camera rig capturing stationary objects. The method according toaspects of the invention may be performed automatically orsemi-automatically. This will speed up the inspection process and leadto savings in time and expenses.

According to an advantageous embodiment of the invention, at least onemotion vector and/or the disparity is determined by two dimensionalblock matching or by feature point tracking. The displacement of theobject is preferably determined within the first or within the secondvideo stream of the 3D video content. As far as signal processing isconcerned, this task is basically the same as disparity estimationbetween corresponding images of stereoscopic video content. However,disparity estimation typically has a preferred horizontal searchdirection. Accordingly, for example a two dimensional disparityestimator based on block matching may be applied. However, this twodimensional “disparity” estimation is computationally demanding and maysuffer from ambiguities. Feature point tracking seems to be moresuitable and is computationally less demanding, because of the limitednumber of points which has to be examined. Also a mixture of the twoapproaches may be applied. The ambiguities of the two dimensional blockmatching may for example be resolved using descriptors similar to thoseemployed in feature point tracking.

The synchronization mismatch may be determined based on a verticalcomponent of the motion vector and the corresponding disparities,according to another advantageous aspect of the invention. However, thissimple determination of the synchronization offset is restricted todetected motion vectors having a vertical component which issubstantially greater than zero. Analyzing the vertical component of thedisparity has the considerable advantage that 3D video content shouldideally not exhibit any vertical disparities. If vertical disparitiesexist, it is a bad sign anyway. If there are vertical disparities whichare due to erroneous synchronization issues, the frame offset, i.e. thetemporal synchronization mismatch between the two captured videostreams, may be determined with sub frame accuracy. This may beperformed by simply dividing the measured vertical disparity between thedifferent camera views through the vertical component of the motionvector, which is for example due to a displacement of an object, betweensubsequent images of one camera, i.e. between subsequent frames of onevideo stream.

However, vertical disparities can also exist for other reason, forexample due to a misalignment or a convergence of the cameras of thestereo camera rig. However, these disparities may be assumed to beconstant in time. Since a deviation in the motion vector and acorresponding deviation of the disparity are used to determine atemporal synchronization mismatch, this static vertical disparity fieldis disregarded. Alternatively, for a calculation of the temporalsynchronization mismatch, the static vertical disparity field may besimply subtracted from the measured values before calculation of theframe offset i.e. before calculation of the synchronization mismatch.

However, vertical motion is typically less common and also smaller thanhorizontal motion, in most video content. This restricts theapplicability and the accuracy of the mentioned analysis of verticaldisplacements.

According to another aspect of the invention, for a motion vector havinga horizontal component which is substantially greater than zero, a typeof camera and object movement is determined. Within the context of thespecification, a camera movement is a displacement of a camera, forexample a tracking shot or dolly shot. Further, a camera movement shallbe a camera panning, too. Also a camera zoom which means a zoom in or azoom out should be referred to as a camera movement. Camera movementsmay be determined by analyzing the vector field for objects or points inthe captured frames. There are well known typical vector fields for eachof the above-mentioned camera movements. For example, for a trackingshot or a camera pan, nearly all pixels move with a same speed anddirection.

If a camera pan is detected, the synchronization mismatch may bedetermined by dividing the deviation in the horizontal component of thedisparity by the variation of the horizontal component of the motionvector. In an ideal case of a camera pan, there is a horizontal rotationof the camera around the focal point and all pixels in the capturedframes move with the same speed and disparities stay constant.Therefore, any erroneous change in disparity will be related to asynchronization offset and may be determined as easily as for verticalmotion.

Further, according to another aspect of the invention, if the motionvector has a horizontal component which is substantially greater thanzero and the determination of the camera movement indicates a horizontalcamera displacement, for example a tracking shot, the synchronizationmismatch is determined by dividing a product of the deviation indisparity and a base line of a stereo camera rig which has been appliedfor capturing the 3D content by the product of a speed of the cameramovement and the disparity in the first number of frames. For a purehorizontal translation of the camera rig, the synchronization offset hasthe same effect as a change in the base line between the two cameras ofthe stereo camera rig. A relative change in disparity is thereforerelated to a virtual change in the base line via the speed of the cameramotion and the synchronization offset. For example, if the disparitiesare reduced by 10%, the camera motion multiplied with a synchronizationoffset will be equal to 10% of the base line between the cameras of thestereo camera rig.

According to another aspect of the invention, for a motion vector havinga horizontal component which is substantially greater than zero, a typeof object movement is determined. This object movement may be determinedby analyzing the vector field. Typically, a limited group of pixels in acertain area is moving while the remaining pixels are at rest. If theobject movement is a fronto parallel movement, the synchronizationmismatch is determined by dividing the variation of the horizontaldisparity by the deviation of the horizontal component of the motionvector of the object.

It is often possible to determine the synchronization mismatch byassuming that the true change in disparity between two consecutiveimages of one video stream is negligible when compared to the erroneouschange in disparity which is caused by the temporal synchronizationmismatch. A fronto parallel movement may be assumed if the change insize of an object is much smaller than its change in position due to adisplacement in the real world. This may be explained by makingreference to the geometry of a stereo camera rig. For a stereo cameraarrangement of two parallel ideal pinhole cameras, this is explained forexample in M. Brown et al.: “Advances in Computational Stereo”, IEEETrans. on Pattern Analysis and Measuring Intelligence (PAMI), Vol. 25,No. 8, 2003. As a consequence, if the apparent motion of an object ismuch larger than its change in size, this is even more true for itschange in disparity. For such fronto-parallel motion, the disparitiesmay safely be assumed constant.

However, there may be outliers in the determined disparity values. Thesemay be due to an inaccuracy during determination of feature points.

According to further aspects of the invention, a plurality of motionsvectors for a plurality of groups of pixels may be determined forconsecutive frames of the left and/or the right video stream. Further, aplurality of disparities between the left and/or the right video streammay be determined for said groups of pixels. The motion vectors and thedisparities are determined for a first number of frames and for asubsequent second number of frames of the left and the right videostream, respectively. Subsequently, the motion vectors in the firstnumber of frames may be compared with the motion vectors in the secondnumber of frames and a variation of motion vectors may be detected. Upondetection of said variation, the disparities which have been determinedfor the first number of frames are compared with the disparities whichhave been determined for the second number of frames and a preliminarysynchronization mismatch is determined by comparing the deviation in therespective disparities. To tackle the problem of outliers, a median ofsaid preliminary synchronization offsets is calculated.

The mentioned calculations may be performed on a per-frame basis whichmeans that the disparity value is determined for a pair of stereo imagesand the motion vector is determined for two subsequent frames within oneof the video streams. However, it is also possible to sum up thedisparity values for a plurality of frame pairs and to divide this sumby the sum of the motion vectors for a plurality of subsequent frames.The latter offers an averaging effect and will probably lead to moreconsistent results. Robustness of the determination of the motion anddisparity values may again be further improved by calculating a medianof the preliminary determined values.

According to another aspect of the invention, a video processingapparatus for detecting a temporal synchronization mismatch between atleast a left and a right video stream of 3D video content is provided.The video processing apparatus is configured to determine a motionvector a group of pixels in consecutive frames of the left and the rightvideo stream. Further, the video processing apparatus is configured todetermine a disparity for said group of pixels in the left and the rightvideo stream. The motion vector and the disparity are determined for afirst number of frames and a subsequent second number of frames of theleft and the right video stream, respectively. The motion vector in thefirst number of frames is compared with the motion vector in the secondnumber of frames. Upon detection of a variation of the motion vector,the video processing apparatus is configured to compare the disparitywhich has been determined for the first number of frames with thedisparity which has been determined for the second number of frames,wherein a deviation of the disparity is indicative to thesynchronization mismatch.

Same or similar advantages which have been already mentioned for themethod according to aspects of the invention apply to the videoprocessing apparatus according to aspects of the invention in a same orsimilar way and are therefore not repeatedly mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding the invention shall now be explained in moredetail in the following description with reference to the figures. It isunderstood that the invention is not limited to this exemplaryembodiment and that specified features can also expediently be combinedand/or modified without departing from the scope of the presentinvention as defined in the appended claims. In the figures:

FIG. 1 is a simplified set of frames illustrating a moving object in aleft and a right video stream of 3D video content,

FIG. 2 is a further simplified set of frames illustrating the movingobject in a left and right video stream of 3D video content, whereinthere is a temporal synchronization mismatch between the left and rightvideo stream,

FIG. 3 is a frame showing a moving object at several time instances in asingle comprehensive frame of a left and a right video stream ofstereoscopic video content,

FIG. 4 is a further comprehensive illustration of the complex movement,wherein there is a constant horizontal disparity due to a tightsynchronization of the left and right camera,

FIG. 5 is a simplified set of frames illustrating the movement of theobject of FIG. 3 in a frame sequence, wherein there is a synchronizationmismatch between the left and right camera,

FIG. 6 is a further set of simplified frames illustrating an objectwhich starts to move, wherein there is a synchronization mismatchbetween the left and right camera, and

FIG. 7 is a simplified video processing apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a simplified sequence of frames showing a moving object, i.e.the image of the moving object 2L in a left video stream L and an imageof this moving object 2R in a right video stream R of 3D video content.By way of an example only, the embodiment refers to a moving objectwhich moves along a trajectory. The trajectory is illustrated by amotion vector 4 in the depicted frames which is a sequence of frames forthe points in time T=0 to T=2. Due to the slightly different perspectiveof the left and right camera of a set of stereo cameras which capturethe left and right video stream L, R, the image of the moving object 2L,2R has slightly different positions in the respective pairs of frames.In the frames of the right channel R, the position of the object 2L inthe left channel L is represented by an object 2L drawn in dashed line.The difference in the position of the object 2L in the left channel Land the position of the object 2R in the right channel R is thedisparity 6. According to the example in FIG. 1, the object moves frontoparallel and the left and right camera of the stereo camera rig aretightly synchronized. Accordingly, the disparity 6 is a constant vectorfor each pair of frames. In other words, for every moment in time (T=0 .. . T=2), the disparity 6 is a constant vector.

In FIG. 2, there is a sequence of frames showing the same moving objectas in FIG. 1. However, this time there is a synchronization mismatchbetween the left and right video stream L, R. Accordingly, the disparity6 between the left image of the object 2L and the right image of theobject 2R differ by a disparity 6 which is different compared to thedisparity 6 in the sequence of frames in FIG. 1. The disparity vector 6is slightly greater than the disparity vector 6 in the sequence of FIG.1, because the synchronization mismatch is +Δt for the right videostream R. Further, due to the fact, the motion vector 4 of the objecthas a horizontal and a vertical component there is a vertical componentin the disparity vector 6, too. A vertical component of the disparity 6is a strong hint to a synchronization mismatch and vertical disparitiesare well known to cause visual fatigue and are therefore undesired in 3Dvideo content. However, despite of the fact that the vertical componentof the disparity 6 is a hint for a temporal synchronization mismatch, itis not possible for the shown example to safely determine thesynchronization mismatch by analyzing the disparity 6 alone. It isimpossible to discriminate between the assumed scenario that there is asynchronization mismatch between the left and right video stream L, Rand a misalignment between the left and right camera (capturing the leftand right video stream L, R) of the stereoscopic camera pair. This isbecause the vertical component of the disparity 6 is constant over timeand may be due to a negative tilt of the right camera, for example.

In order to eliminate this ambiguity, the method according to anembodiment of the invention compares the motion vector 4 of the object2L, 2R in a first sequence of frames with the motion vector 4 of theobject 2L, 2R in the second sequence of frames. By analyzing thevariation of the motion vector 4 a temporal synchronization mismatch maybe determined. For example, the object 2L, 2R may perform a steadymovement in the first sequence of frames and may change the motionvector 4, for example by acceleration or deceleration or by a change ofdirection of movement, in the second sequence of frames.

In FIG. 3, there is an object 2L, 2R performing a steady movement duringa first number of frames, i.e. the first three frames (T=0 . . . T=2)and which accelerates and changes its direction of movement in thesecond number of frames, i.e. the following frames (T=3, T=4). For thesake of clarity, the respective positions of the image of the object 2Lin the left video stream L and in the right video stream R are presentedin one single frame only. Further, for clarity reasons, only some of theobject images 2L, 2R are given reference numerals. The change inmovement of the object 2L, 2R is clear from the motion vector 4 whichchanges over time. In the right video stream R, the position of theobject 2L in the left video stream L is illustrated by an object 2Ldrawn in dashed line.

First, it shall be assumed that the left and right camera is tightlysynchronized. Accordingly, the disparity 6 between the image of theobject 2L in the left channel L and the image of the object 2R in theright channel R is constant over time, as it is illustrated in FIG. 4.

In the image sequence of FIG. 5, there are a plurality of frames of theleft video stream L and the right video stream R for a plurality ofpoints in time (T=0 . . . T=3). The object 2L, 2R performs the movementwhich is known from FIGS. 3 and 4, however, there is a temporalsynchronization mismatch between the left and right camera.

At T=0 and T=1, the object 2L, 2R is in uniform motion and accordingly,the motion vector 4 is constant over time. The disparity 6 comprises avertical component due to the synchronization mismatch between the leftvideo stream L and the right video stream R. At T=2, the object 2L, 2Raccelerates and departs from uniform motion also with respect to itsdirection. Accordingly, the motion vector 4 at T=2 has a differentdirection and a greater absolute value. The corresponding frames of theleft video stream L and the right video stream R at T=2 have a disparity7 which is different from the disparity 6 for the frames at T=0 and T=1.This is due to the temporal synchronization mismatch of +Δt for theright video stream R. The difference between the disparity 6 in thefirst number of frames (i.e. in the frames at T=0 and T=1) and thedisparity 7 in the second number of frames (i.e. in the frames for T=2and T=3) is indicative to the synchronization mismatch between the leftvideo stream L and the right video stream R. At T=3, the objectcontinues to move fast and again, a uniform motion may be present.Accordingly, the new disparity 7 is constant in the second number offrames.

FIG. 6 is a further sequence of simplified frames showing an image of anobject 2L in the left channel L and the corresponding image of theobject 2R in the right channel R. In the pairs of frames at T=0 and T=1,the object 2L, 2R is at rest and accordingly, there is a constantdisparity 6 between the left image of the object 2L and the right imageof the object 2R. However, at T=1, the object 2L, 2R starts to move andin the pair of frames at T=2 there is an additional disparity 8 which isdue to the movement of the object 2L, 2R and the synchronizationmismatch between the left video stream L and the right video stream R.This additional disparity 8 is the difference between the disparity 6for the first number of frames (i.e. for the frames at T=0 and T=1) andthe disparity 7 in the second number of frames (by way of an example,this second number of frames comprises the pair of frames at T=2 only).Again, the synchronization mismatch may be determined by analyzing thechange of the motion vector 4 (which changes from zero to a valuesignificantly greater than zero) in two frames and by comparing it withthe disparity values for these two frames.

FIG. 7 is a simplified video processing apparatus 10 comprising aprocessing unit 12 for performing the method according to aspects of theinvention. Further, the video processing apparatus 10 comprises adisplay unit 14. The video processing apparatus 10 may be configured inthat the processing unit 12 receives 3D video content (3D-AV) andperforms an automated detection of a temporal synchronization mismatchin the 3D video content (3D-AV). For quality control, the videoprocessing apparatus 10 may be configured to display a result of thedetection of the temporal synchronization mismatch at the display unit14 and an optional reproduction of the video content. Accordingly, anoperator may check the quality of the automated synchronization mismatchdetection and if necessary may adjust the synchronization mismatchmanually.

Although the invention has been described hereinabove with reference tospecific embodiments, it is not limited to these embodiments and nodoubt further alternatives will occur to the skilled person that liewithin the scope of the invention as claimed.

1. A method for detecting a temporal synchronization mismatch between atleast a left and a right video stream of 3D video content, the methodcomprising the steps of: determining a motion vector for a group ofpixels in consecutive frames of the left or the right video stream,determining a corresponding disparity for said group of pixels in theleft or the right video stream, wherein the motion vector and thedisparity are determined for a first number of frames and a subsequentsecond number of frames of the left or the right video stream, comparingthe motion vector of the group of pixels in the first number of frameswith the motion vector of said group of pixels in the second number offrames and upon detection of a variation of the motion vector: comparingthe disparity of the group of pixels which has been determined for thefirst number of frames with the disparity which has been determined forthe second number of frames, wherein a deviation of the disparity isindicative to the synchronization mismatch.
 2. The method according toclaim 1, wherein the motion vector or the disparity is determined by twodimensional block matching or by feature point tracking.
 3. The methodaccording to claim 1, wherein for a variation of the motion vectorhaving a vertical component which is substantially greater than zero,the synchronization mismatch is determined by dividing the verticalcomponent of the deviation in disparity by the variation of the verticalcomponent of the motion vector.
 4. The method according to claim 1,wherein for a variation of the motion vector having a horizontalcomponent which is substantially greater than zero, the method furthercomprises the steps of: determining a type of camera movement, whereinfor a camera pan, the method further comprises the step of: determiningthe synchronization mismatch by dividing the deviation in the horizontalcomponent of the disparity by the variation of the horizontal componentof the motion vector.
 5. The method according to claim 1, wherein for avariation of the motion vector having a horizontal component which issubstantially greater than zero, the method further comprises the stepsof: determining a type of camera movement, wherein for a horizontalcamera displacement, the method further comprises the step of:determining the synchronization mismatch by dividing the product of thedeviation in disparity and a base line of a stereo camera arrangementwhich has been applied for capturing the 3D video content by the productof a speed of the camera movement and the disparity in the first numberof frames.
 6. The method according to claim 1, wherein for a variationof the motion vector having a horizontal component which issubstantially greater than zero, the method further comprises the stepsof: determining a type of movement for an object, wherein for a frontoparallel movement of the object, the method further comprises the stepof: determining the synchronization mismatch by dividing the variationof the horizontal disparity by the deviation of the horizontal componentof a motion vector of the object.
 7. The method according to claim 1,further comprising the steps of: determining a plurality of motionsvectors for a plurality of groups of pixels in consecutive frames of theleft or the right video stream and determining a plurality ofcorresponding disparities for said plurality of groups of pixels in theleft or the right video stream, wherein the motion vectors and thedisparities are determined for a first number of frames and a subsequentsecond number of frames of the left or the right video stream, comparingthe motion vectors for said plurality of groups of pixels in the firstnumber of frames with the motion vectors of said plurality of groups ofpixels in the second number of frames and upon detection of a variationof the motion vectors: comparing the disparities which have beendetermined for the first number of frames with disparities which havebeen determined for the second number of frames, wherein a preliminarysynchronization mismatch is determined by comparing the deviation of thedisparities with the variation of the motion vectors and calculating amedian of said preliminary synchronization mismatches so as to determinethe overall synchronization mismatch.
 8. A video processing apparatusfor detecting a temporal synchronization mismatch between at least aleft and a right video stream of 3D video content, wherein the videoprocessing apparatus is configured to: determine a motion vector for agroup of pixels in consecutive frames of the left or the right videostream, determine a disparity for the group of pixels in the left or theright video stream, wherein the motion vector and the disparity aredetermined for a first number of frames and a subsequent second numberof frames of the left or the right video stream, compare the motionvector for said group of pixels in the first number of frames with themotion vector) for said group of pixels in the second number of frames,wherein upon detection of a variation of the motion vector, the videoprocessing apparatus is configured to: compare the disparity which hasbeen determined for the first number of frames with the disparity whichhas been determined for the second number of frames, wherein a deviationof the disparity is indicative to the synchronization mismatch.